1. Field of the Invention
The invention relates to a method and an apparatus for isolating mechanical vibrations reacting to metrology tools, in particular beam metrology tools.
2. Description of Related Art
In particular, when using a beam metrology tool in industrial fabrication, often the environment is not optimal for the incorporated metrology instrument, in particular regarding the high resolution metrology using electron beam metrology instruments for example.
As known, such electron beam metrology instruments based on the principle of Scanning Electron Microscopes (SEM) are focusing an electron beam which scans an area of a certain specimen. As a result, certain reaction effects of the electron beam hitting the specimen are detected, whereby the intensity of the detected signal can be used to create an image of the scanned area.
In addition to generic SEM, an electron beam metrology instrument often is specialised as quality control tool in semiconductor or in general micro-technology fabrication. There the specimen, such as an chip or wafer, contains a larger number of measure points. In order to work the different measure points usually a chuck supported by a sophisticated mechanism, normally named stage, is used for positioning the specimen.
A major requirement of those instruments towards vibration isolation is to allow those instruments to achieve the necessary measurement resolution in spite of high disturbance levels.
Environmental disturbances of concern are on the one hand side electromagnetic disturbances, which deflect the electron beam from its intended path, and on the other hand side mechanical vibrations due to floor vibration and/or due to acoustic excitations.
Besides electromagnetic disturbances, mechanical vibrations due to floor vibration often can be reduced effectively by conventional vibration isolation principles where an isolating layer can be inserted into the path of the disturbance from the floor via the supporting structure to the actual measurement tool.
However, in particular mechanical vibrations due to acoustic excitations are much harder to influence as the disturbances directly affect the mechanical supporting structure, providing for example an encapsulating vacuum chamber for the electron beam generating unit as well as for a stage positioning unit. Usually, in practice at least the electron beam and the specimen are enclosed in a vacuum.
Moreover, even if a supporting or outer structure providing a vacuum chamber and hence, enabling the fact that the measurement takes place in vacuum, appears to be ideal to isolate from acoustic excitation, the vacuum chamber usually does not merely bear as function to allow vacuum but unfortunately also to give a stiff structure between electron beam on one side and specimen on the other side.
Trying however to use low stiffness elements in order to mechanically de-couple the chuck and the electron beam from the enclosing vacuum chamber leads into disadvantages concerning the positioning accuracy between specimen and chamber and hence, is a drawback to measurement resolution by itself.